Our start battery worked flawlessly, during our week-long summer cruise. Two weeks later, when I pressed the start button there was a loud click followed by silence. Only after disassembling the engines electronics (or more accurately electromechanics), did I think of checking the battery.
The battery was connected to a charger, and appeared to be receiving maintenance charge. However, the voltage dropped, to 4 volt when attempting to start. In the two weeks the battery was left idle, but charging, it had developed a major internal resistance. I am guessing it is a dried out cell. ‘Maintenance free’ should read ‘can-not-be-maintained’.